Heart rate sensors usually work best when the watch is worn tightly. Most people tend to wear their watch too loosely on their wrist, which lets in a lot of background light. Since these sensors are optics-based, that light translates to interference. Try wearing it one notch tighter than you usually do (or slightly higher on your wrist, if tightening isn't an option for you), and see if that makes a difference.

Also, for what it's worth, accuracy isn't as important as consistency. If one device consistently reads you at 120 BPM and another consistently reads you at 130 BPM during the same activities, you at least know that you're getting the same (albeit slightly scaled) results. As with most things in this space (quasi-medical equipment), most readouts are going to be an algorithmic estimate, as opposed to a true live reading.

My Garmin heartrate monitor is spot-on. I'm sure that's not unusual.

I'm not sure how old qualifies for this post, but loads of Garmins are out there 5+ years old and working just as well as the day they were made. Mine is only 4.5 years old and runs perfectly.

I run into much the same thing, although newer Apple Watches may be more reliable. So long as I only use it to spot trends and not to detect real-time events, a Garmin Vivoactive 3 seems to work pretty well.